Photoelectric devices generate an electrical signal in response to received electromagnetic radiation, such as visible, infrared, or ultraviolet radiation. Photodetectors such as CCDs, photodiodes, phototransistors, focal plane arrays, and photovoltaic (PV) cells are all examples of photoelectric devices.
Accurate characterization of the optical spectral response of photoelectric devices is needed for certain applications, for example, for solar applications. In solar applications, knowledge of PV conversion efficiencies is important in determining and expanding their performance across a wider spectral range. For example, developments in new materials that can absorb light from the ultraviolet through the infrared offer unforeseen possibilities in terms of conversion efficiencies and applications. Accurate characterization of the electrical response of the PV devices for such materials is often central to this objective.
As commonly used, the term “solar cell” is reserved for devices intended specifically to capture energy from natural sunlight, while the term PV cell is used when the light source is unspecified. A primary parameter describing the performance of a solar cell device is the power-conversion efficiency, as evidenced by the Watt-peak rating. The most common procedure employed to characterize a solar cell device is the current-voltage (I-V) response under simulated sunlight illumination. By knowing the incident power, the power-conversion efficiency of the device can be calculated.
The short-circuit current density (Jsc) is a measure of the photocurrent arising with no external bias applied with the device electrodes connected via an external circuit. The open-circuit voltage (Voc) represents the necessary externally applied voltage for which no net current passes through the solar cell device. The point along the I-V response curve at which the product of the voltage and current provides a maximum is the point of maximum power and represents the optimal output driving load for the solar cell device. However, in addition to the intensity of the illumination source, it is important to account for the spectrum of the illumination source and its difference from the standard solar spectrum to obtain the “true” power efficiency of the solar cell device.
A reference cell is commonly used to establish the intensity of the illumination source used for testing. However, there may be error in the measured Jsc of the test device arising from differences in the spectrum provided by the illuminator and/or differences in the spectral response of the reference cell and the PV device under test, (PV-DUT), such as a solar cell.
The spectral response of a PV-DUT is generally measured by exposing the PV-DUT to monochromatic light that is scanned through the wavelength range of interest. The amount of output signal (e.g., output power) resulting from exposure to the monochromatic light at each wavelength increment is first measured with a standard (reference) photodetector having a known photoelectric response in a light path, and then the PV-DUT is placed in the same light path. The signal from the PV-DUT divided by the amount (power) of light obtained from the reference photodetector is the response of the PV-DUT. The monochromatic source is typically a scanning grating monochromator. A scanning monochromator uses precision mechanics and slits to select specific wavelengths dispersed by gratings from a broadband spectrum.
The resulting power of monochromatic light from such a source is very small. It has also been observed that many solar cell devices respond differently at high light levels as compared to low light levels. While it is impractical to generate tunable monochromatic light spanning the visible spectrum (and beyond) at the same brightness as the sun under this method, a broadband or “white light bias” source can be used to simulate a bright environmental condition. This is a constant light source that raises the overall energy level in the test sample to approximate internal conditions representative to the measure of the solar cell's spectral performance regime under real world conditions.